There have been proposed in-mold coating formation methods, aiming at improving a quality of products by adding some additional values such as decorative effects on some thermoplastic resin molded products often used in automobiles and house hold electric apparatus or devices or used as construction materials, or aiming at reducing the production cost by omission of certain steps in molding process thereof; said in-mold coating formation method comprises the steps of injecting a coating material into a space between the surface of a thermoplastic resin molded product formed in a mold and the internal surface of the cavity of the mold, and then curing the coating material within the mold so as to obtain an integrally formed molded resin product on whose surface a coating layer is adhered. In particular, those methods have been used to certain extent in the production of the thermoplastic resin products used as various parts in an automobile vehicle whose requirements as to the appearance and the quality are quite severe. These parts may include a bumper, a sideview mirror cover, a fender, a door panel, a back door panel, an over fender, a door handle, a side garnish, a side protector, a wheel cap, including a side cover and a cowl for use in a motorcycle.
There have been known several in-mold coating formation methods, such as those disclosed in U.S. Pat. No. 4,076,788, U.S. Pat. No. 4,081,578, U.S. Pat. No. 4,331,735, U.S. Pat. No. 4,366,109, U.S. Pat. No. 4,668,460, JP-A-5-301251, JP-A-5-318527, and JP-A-8-142119.
In the case of methods disclosed therein, there are some prescriptions as to the mold closing force at the time when a coating material is injected into the space between the inner surface of the mold and the thermoplastic resin molded product after formation of the molded products from the thermoplastic resin material for molding within the mold, the injection pressure of the coating material, and a mold clearance. However, there is almost no attention to be paid to time required for injecting a coating material and the time required for completing the reclosing of the mold after the coating material has been injected into the mold.
Namely, a thermosetting material starts its solidification by virtue of a heat of the inner surface of the mold and a heat of the thermoplastic resin material immediately after it is injected into a mold. A curing speed thereof will vary from case to case, depending upon various conditions such as the sort of a coating material, the temperature of a mold, the temperature of a thermoplastic resin material, and the like.
If the coating material injection time is too short, pigment components contained in a coating material will be undesirably separated from each other in the coating material or some weld lines will be undesirably formed. On the other hand, if the coating material injection time lasts too long, an end portion of the molded product is often not coated since a flowability of a coating material will decrease with the progress of the solidification of the coating material, and/or some wrinkles and cracks will be formed in a coating layer.
Moreover, if an operation time until the completion of the reclosing of the mold is too long, the coating material will undergo an undesired gel with the progress of its solidification, resulting in a decrease in its flowability and hence making it difficult for a coating material to cover an entire molded resin product including its end portions, and/or a pressure needed in reclosing the mold will also be applied to the coating material during its gel, causing some wrinkles and cracks in the cured coating layer formed on the surface of the molded resin product. On the other hand, if an operation time before the completion of the reclosing of the mold is too short, pigment components contained in the coating material will be undesirably separated from each other in the coating material, and some weld lines will be undesirably formed, making it impossible to produce molded resin products having a uniform appearance. Further, in the case of molded products having ribs and bosses, if a pressure for reclosing the mold is not made proper, some defects such as sinks and humps (which are in fact tubercles formed on the surface of thick portions of the molded product) will be formed in the molded products.
For this reason, firstly, it is earnestly desired at the present time to establish an improved in-mold coating formation method which requires that after a thermoplastic resin product has been formed within a mold, a coating material is injected into the mold to form a coating layer on the surface of the molded resin product, with the coating formation process being carried out without forming any wrinkles, cracks, mottles, and weld lines in the cured coating layer, thereby obtaining molded resin products having a coating layer of a high quality.
Further, the above mentioned in-mold coating formation method (hereinafter it is referred to sometimes as IMC method) has attracted a considerable attention from people in the art and has been considered to be very effective to serve as a substitution for a conventional spray coating technique; this is because there is a tendency that official restrictions for controlling the discharge of harmful organic compounds from various factories into surrounding atmosphere become more and more severe, and from the viewpoints that serious attention should be paid to the health protection of workers working in the factories under the circumstances that environment problems have attracted big concerns more and more during recent years.
Incidentally, the above mentioned IMC method was initially developed mainly for use in manufacturing a molded resin product from a thermosetting resin such as SMC or BMC. However, in recent years, attempts have been made to apply an IMC method to form an thermoplastic resin product. For example, as is disclosed in JP-A-5-301251, there has been proposed a method comprising injecting a coating material of a thermosetting resin onto the surface at the injection inlet of a molded resin product by altering a closing force of the mold, or maintaining a closing force of the mold at a constant level, under a condition where the surface temperature of a resin is equal to or higher than the curing temperature of a coating material, and opening the mold after the coating material has been cured.
However, it is difficult to design optimum molding conditions for a formed coating layer to obtain a good appearance and a good adhesion strength, compared with an IMC method for a thermosetting resin from the following reasons or the like: a mold temperature in an IMC method for a thermoplastic resin will be fairly lower than that in an IMC method for a thermosetting resin; and, in the case of a coating material for a thermoplastic resin in an IMC method. it is required to have a curing property capable of curing at a temperature lower than a coating material for a thermosetting resin does.
Further, a conventional injection molding machine is designed only for molding a resin product having a predetermined shape, but not for carrying out an IMC method. Therefore, one may point out the point, as one of reasons making it difficult to use an IMC method of a thermoplastic resin, that the conventional injection molding machine is designed not so as to carry out the controls of the position of a mold and the mold closing force with a high precision and a high response. That is, it is impossible to spread a coating material sufficiently within the cavity of a mold after the injection thereof into the mold, or it is extremely difficult to obtain a uniform coating layer since an injected coating material starts to cure partially soon after it has been injected into the mold. Accordingly, even if in a case where the method disclosed in JP-A-5-301251 is used, it is difficult to control the curing condition for curing the coating material, as far as a conventional injection molding machine is used whose control operation for controlling the mold closing force and mold position is slow, hence rendering it difficult to ensure a high productivity.
Therefore, in order to improve the above mentioned situation, JP-A-6-254886 has proposed an attempt to adjust conditions for an IMC method by giving a predetermined amount of opening of the mold. However, in the case of the IMC apparatus for this method, there is employed a means for stopping at a position capable of ensuring a predetermined amount of opening of the mold as a result of the interaction between two sets of the driving means installed in the opposite direction each other, by installing another driving means for driving the mold in the opposite site in addition to a driving means for effecting the opening or closing of a mold. Accordingly, the control system thereof is complex and it is difficult to ensure a high response. This brings inherently a problem that it is impossible to shorten an operation time which lasts until the mold arrives at its predetermined stop position.
Moreover, with regard to an injection molding machine of a hydraulic direct press type which has been used in prior art, since a mold closing force control for controlling a mold pressing force and a mold opening amount control for controlling the mold position are effected by different control systems, although very slightly, a time lag will occur when the mold position control is changed over to the mold closing force control, making it impossible for the mold to be suitable for a continuous operation. As a result, with regard to an IMC method that is carried out in an injection molding machine of a hydraulic direct press type, at the time when the operation of the mold is interrupted, some flow lines will occur in coating material flowing areas within the cavity of a mold. Namely, there has been existing a problem that the conventional injection molding machine of the hydraulic direct press type is not suitable for carrying out an in-mold coating formation method which requires that the mold closing force and the mold opening amount be controlled continuously. In conclusion, there was only a low yield even if people tried their best to manufacture a thermoplastic resin molded product coated with a properly formed coating layer.
Because of the reasons stated in the above, it has been strongly demanded to develop an improved in-mold coating formation method and an improved in-mold coating formation apparatus, which are able to control a mold closing force and a mold opening amount with a high precision and a high response even under a condition where the mold closing force and the mold opening amount are required to be continuously changed, so as to greatly enlarge a selectable range for selecting suitable manufacturing conditions for carrying out an IMC method for a thermoplastic resin so as to produce an integrally formed resin product being excellent in the appearance and the adhesion strength of its coating layer.
However, as discussed in the above, although the conventional IMC method is utilized partly only for the manufacture of a molded product from a thermosetting resin such as SMC resin and BMC resin, this method has been not utilized widely yet for an injection molding of a thermoplastic resin. A key reason for this, one may point out one problem that the coating material often leaks out of the mold. In particular, since it is not easy to carry out an operation to remove coating material leaked from the mold, and since the injection molding machine has to be stopped at every time when the coating material is leaked, a cycle for the formation of a molded product will become too long, hence resulting in a low productivity. Moreover, since the leaked coating material can form an additional load during a process in which a mold is being closed, a predetermined mold closing force will become insufficient, causing a problem that the leaked coating material will adhere to a molded product to be produced in a next production cycle, thus making it difficult to constantly maintain a good quality for molded resin products.
In order to cope with the leakage problem of a coating material, JP-A-6-328505 has proposed an improved mold for use in an injection molding process, in which the shear edge portion(s) is(are) formed so as to prevent a coating material from leaking out the mold, while utilizing parting surfaces. However, since it is still impossible to completely prevent a leakage of a coating material even if this type of the mold is employed, an internal space is required to be formed within the mold for storing the leaked coating material. Consequently, since it is necessary to perform an operation to remove the coating material accumulated in the shear edge portion(s) and this internal space, it is considered that this is the cause of reducing the productivity.
In addition, there is proposed in JP-A-9-48044 a mold having parting surfaces wherein an auxiliary cavity is arranged in parallel with the parting surfaces in order to prevent a possible leakage of a coating material. However, in the case that this type of the mold is used, the coating material can be injected into the mold only under the state where the mold is closed.
Further, the said publication also discloses another mold having grooves formed within the auxiliary cavity. However, such grooves are required to have a thickness of 0.1 to 0.5 mm not so as to make a coating material leaked into a clearance (a gap formed between a groove-forming rib area and a mold cavity surface) formed due to a curing shrinkage of a resin material used for molding. One may not allow, however, to have a sufficient height in the case of the grooves having such a thickness due to the requirements in the strength of the mold movement. Consequently, if a coating material is injected into the mold by opening it at a predetermined opening amount, a coating material will be leaked. On the other hand, even if the grooves are made higher than the opening amount of the mold, there is still a possibility that the grooves are not durable enough in their strength against an injection pressure of coating material.
Moreover, there is disclosed in JP-A-9-52262 a mold being formed with a recess portion located surrounding an opening portion (sprue portion) of a molten resin injecting section, so as to prevent a coating material from flowing into the sprue portion. However, similar to an invention disclosed in JP-A-9-48044, when a coating material is injected into a mold opening it at a predetermined amount, it is considered that the effect of preventing a possible leakage of a coating material is small.
That is, thirdly, there is a strongly desire to provide an in-mold coating formation mold which is capable of preventing a coating material from leaking out of the mold and an in-mold coating formation method using the same, so as to shorten the formation cycle of each molded product and at the same time to stabilize the product quality.
Moreover, in an IMC method for a thermosetting resin, a mold to be used is mainly a shear edge type mold. This is because the coating material is prevented from flowing out of the mold cavity at the time of injecting subsequently the coating material since the thermosetting resin will exhibit so good flowability at the early stage of the molding that the resin material can fill the clearance within the shear edge portion(s).
On the other hand, in an IMC method for a thermoplastic resin, although there has been used an injection molding process suitable for molding a thermoplastic resin, most of the molds for use in the method are flat parting type molds not having shear edge portions.
However, in the case when this flat parting type mold is used, parting surfaces of the mold are not sealed by a previously injected resin to be used for molding, so that the sealing of a coating material at the end portions of a mold cavity is not sufficient. As a result, since the coating material will leak from the mold cavity, one may not keep the coating material at end portion of the mold under a high pressure. Accordingly, there has been such a problem that the quality of the coating layer of an integrally formed molded product is injured. This is because the adhesion strength between the coating material and an thermoplastic resin molded product is reduced due to the insufficient pressure at the vicinity of the end portions of the integrally formed molded resin product, in the case of the IMC method for a thermoplastic resin by using a conventional flat parting mold as mentioned above.
In order to solve the above problem associated with the above IMC method for a thermoplastic resin, there has been employed such a practice that there is used a special coating material containing a component having an excellent affinity with a thermoplastic resin, or a modified special grade resin, so as to cover the insufficiency in the adhesion strength of a coating material. However, there has been a problem that the development of a special coating material and a special grade resin not only needs a long time and a considerable expense. Additionally, it has been found to be impossible to completely remove the insufficiency of the adhesion strength of a coating material.
In addition, since a conventional hydraulic direct press type injection molding machine using a hydraulic cylinder has not been so designed to be able to carry out the IMC method, it is difficult to perform a delicate position control on a mold. Further, since the driving speed of the mold is slow, it is difficult to control operating conditions lasting from the injection of the coating material to the curing thereof. Thus, the above-mentioned may be considered to be one of the factors making it difficult to molding of a thermoplastic resin by the IMC method.
That is, fourthly, it has been demanded to provide an IMC method which is capable of producing an integrally formed molded resin product having an improved adhesion strength between a coating layer and the molded resin product formed by a thermoplastic resin material.
As discussed in the above, in recent years there have been tried attempts so as to use an IMC method for molding a thermoplastic resin material. For example, JP-A-5-301251 has proposed a method which comprises injecting a thermo-setting coating material on to the surfaces to be coated by changing a mold closing force, or holding the same mold closing force, under a condition where a resin surface temperature and a mold temperature are all equal to or higher than the curing point of a coating material, opening the mold once the coating material has been cured.
Further, JP-A-5-301251 has disclosed the use of an IMC method which is similar to a common injection molding process, involving a step of injecting a molten resin at a temperature of 280° C. into the cavity of a mold, and using a coating material whose curing temperature is at about 130° C. after the mold temperature is made to 130° C. In fact, the curing temperature of a coating material varies one from another depending upon the nature of each coating material. However, if the mold has to be set at a temperature which is equal to or higher than the curing temperature of a coating material, the heating of the mold and a heating means will become too large in their scales, resulting in a high equipment cost and a high equipment running cost. Further, if a mold is used at such a high temperature, there is a fear that the mold will quickly become deteriorated in its quality.
Particularly, as in the method disclosed in JP-A-5-301251, if a mold is at a high temperature, a thermoplastic resin molded product will be in its soft state. Accordingly, it is necessary that a molded resin product be taken out from the mold only after it has been cooled to a temperature at which its shape can be exactly maintained. In this manner, if the resin molding process involves repeated cooling and heating treatments, a time period needed for one cycle molding process will become too long, hence resulting in a low productivity. In view of the above various problems, not only it is difficult to expect a high productivity by using the method disclosed in JP-A-5-301251, but also one may expect a high cost from equipment-wise and production step-wise.
In order to improve a low productivity resulted from the use of a high temperature mold, such as in the case which involves the use of a conventional method disclosed in JP-A-5-301251, the actual situation at present remains such a condition that one should set the surface temperature of a mold for use in the IMC method, in view of the molding formation cycle and the molding capability, at a value which is lower than the curing temperature of a commonly used coating material.
However, in the case of the coating layer formed under the conditions incapable of satisfying the predetermined requirements for curing, there is a possibility that it is difficult to obtain some desired physical properties. Further, coating materials capable of using are often limited. For these reasons, in view of handling coating materials and physical properties of a coating layer, it is preferred that a coating material for use in an IMC method should have a high curing temperature. However, there is such a situation that it is unavoidable to sacrifice the desired physical properties of a coating layer if a high productivity is considered to be important; and, on the other hand, it is unavoidable to sacrifice the high productivity in order to form a coating layer having sufficient physical properties, in the case of the conventional IMC method for a thermoplastic resin material.
That is, fifthly, it is the present situation that there is a strong desire to provide an IMC method for the thermoplastic resin material which is capable of improving the physical properties of coating layer obtainable with shortening in the mold formation cycle so as to improve the productivity, as a consequence of setting a mold temperature at a value which is lower than a curing temperature of a thermosetting coating material, and being capable of curing the coating material at a predetermined temperature and within a predetermined time period.